Linux router used to redirect traffic from the outside to an internal device, but the internal device needs to respond using another port

VBalestero

11/6/22, 1:12 PM

I am working on a router between an external radio/modem and my device, where the router makes a dial-up connection using the ppp protocol to the radio and sets an iptables rules between the received radio IP and my device's IP.

I have the following routes:

Destination     Gateway            Genmask         Flags Metric Ref    Use Iface
default         192.168.1000.101   0.0.0.0         UG    0      0        0 ppp0
192.168.100.101 *                  255.255.255.255 UH    0      0        0 ppp0
192.168.164.0   *                  255.255.255.0   U     0      0        0 eth0

And here are my following iptables rules:

iptables -A INPUT -j ACCEPT
iptables -A OUTPUT -j ACCEPT
iptables -A FORWARD -j ACCEPT
iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT
iptables -t nat -A PREROUTING -i ppp0 -p tcp -j DNAT --to 192.168.0.110
iptables -t nat -A PREROUTING -i ppp0 -p udp -j DNAT --to 192.168.0.110
iptables -t nat -A POSTROUTING --source 0/0 -o ppp0 -j MASQUERADE

With these rules I can access everything on my device like telnet, web server and some other communications using high ports.

My problem is that I need to connect to my device using a source port with a destination port, and this is not a problem, but my device needs to respond using another port it created for this communication to continue.

For example, I try to make a connection using source port 2000 with destination port 2001 and my device's response returns using source port 21550 and destination port 2000. But the response don't get back to the client connected to the other side of the radio.

By using tcpdump inside my router I saw the device response, but I couldn't figure out how to handle this port changes in the iptables rules to send it back to the client connected to the other side of the radio.

Any help will be greatly appreciated!

0 + 5

linux

router

iptables

Michael Hampton

11/6/22, 1:17 PM

Don't use NAT. Just route normally.

VBalestero

11/6/22, 1:24 PM

Michael thanks for the quick answer. But I do note understand your answer, you mean not to use iptables, only the route table, is that correct?

Ron Maupin

11/6/22, 1:24 PM

NAT is _not_ a substitute for routing. You do not use NAT unless you must (private to public or overlapping addressing). You do not have any reason to NAT since you have non-overlapping, private addressing, so you can simply route and bypass all the problems of NAT.

VBalestero

11/6/22, 1:26 PM

Thanks for the quick answers, I will try it out without iptables.

djdomi

11/15/22, 4:16 AM

Moreover, i see you try to use some ports, please tell us what you are trying to fix (X and Y Problem)

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: Linux router used to redirect traffic from the outside to an internal device, but the internal device needs to respond using another port

TH: เราเตอร์ Linux ใช้เพื่อเปลี่ยนเส้นทางการรับส่งข้อมูลจากภายนอกไปยังอุปกรณ์ภายใน แต่อุปกรณ์ภายในจำเป็นต้องตอบสนองโดยใช้พอร์ตอื่น

RO: Routerul Linux a folosit pentru a redirecționa traficul din exterior către un dispozitiv intern, dar dispozitivul intern trebuie să răspundă folosind un alt port

RU: Маршрутизатор Linux используется для перенаправления трафика извне на внутреннее устройство, но внутреннее устройство должно отвечать, используя другой порт

VI: Bộ định tuyến Linux được sử dụng để chuyển hướng lưu lượng truy cập từ bên ngoài vào thiết bị bên trong, nhưng thiết bị bên trong cần phản hồi bằng cổng khác

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